1. Field of the Invention
This invention relates to a method and an apparatus for an anodic treatment on a surface of a piston used for an internal combustion engine. More particularly, the present invention relates to a method and an apparatus for anodizing an annular surface of the piston.
2. Description of the Related Art
It is well known that a portion of the piston used in the internal combustion engine is placed close to a combustion zone. More particularly the portion of the piston is in contact with relatively hot gases, and therefore, is subject to high-thermal stresses that may cause deformations or changes in the metallurgical structure. This negatively affects functions of the portion.
As a measure against such negative affections, a surface of the piston has been treated by an anodic treatment in order to develop an anodic oxide coating that protects a metal of the piston from undesirable affections of heat. One such apparatus that performs the anodic treatment is disclosed in, for example, a Japan Patent Publication (koukai) No. 9-217200 (incorporated herein by reference). According to the publication, as shown in FIG. 19, the apparatus includes a jacket 101, a lid member 102, a mask socket 103, an O-ring 105, an electrolyte bath 106, a nozzle system 107, a cathode 108, and an anode 109. The jacket bath 101 forms a part of a circulation circuit of electrolyte (reaction medium), and is substantially like a cup shape. The jacket 101 has an opening, which is closed by the lid member 102, at its upper end. A hole in which the mask socket 103 is fitted is formed at the center of the lid member 102. The mask socket 103 is substantially cylindrical in shape, and is provided its lower opening portion with an inwardly projected flange portion. A piston 104 is inversely placed in the mask socket 103. Namely, the piston 104 is inserted into the mask socket 103 from its head portion (piston head).
The O-ring 105 is placed on the flange portion. The O-ring 105 touches a surface of the piston head when the piston 104 is placed in the mask socket 103. Thereby, a portion of the piston not to be anodized is sealed. The nozzle system 107, through which the electrolyte is directed to the piston 104, is placed in the electrolyte bath 106 that is provided in the jacket 101. The cathode 108 is provided at an upper portion of the electrolyte bath 106. The anode 109 is in contact with the piston 104. The apparatus disclosed in the publication thus performs the anodic treatment on an end face of component (piston) that is cylindrical or columnar in shape.
According to the publication, however, since the O-ring 105 touches the surface of the piston head, there is a difficulty in anodizing a limited area defined at a middle portion on a cylindrical surface. That is, for instance, where the anodic treatment on the end face of the component (piston) is unnecessary while the anodic treatment on the limited area at the middle portion on the cylindrical surface is carried out, a masking of a portion of the component (the end face) is required to prevent the end face from being anodized. However, to make a mask portion, a masking process to the end face of the component must be accomplished before putting the component in the apparatus. This causes a decline of working efficiency and processing ability.
The electrolyte upwardly flows to the end face of the component through the nozzle system 107, and then, downwardly moves away from the end face to be drained from the electrolyte bath 106. The electrolyte supplied to the end face meets the electrolyte leaving from the surface, which causes an obstruction to a smooth circulation of the electrolyte. To provide the smooth circulation, a large area for flow of the electrolyte is necessary, and thereby, the size of the apparatus becomes large.